In a process of communication between a terminal (UE, User equipment) and a base station (eNB), a sounding reference signal (SRS, sounding reference signal) is a signal transmitted by the UE to the base station. Content of the SRS is known to both the terminal and the base station. After receiving the SRS transmitted by the terminal, the base station may measure a radio channel between the terminal and the base station according to the received SRS.
In systems such as a long-term evolution advanced (LTE-A, Long Term Evolution Advanced) system, the base station may instruct, through a physical downlink control channel (PDCCH, Physical Downlink Control Channel), the terminal to transmit a sounding reference signal.
The terminal acquires the following SRS configuration parameters to accurately transmit the SRS:
SRS transmission time;
frequency domain position for SRS transmission;
SRS bandwidth;
SRS hopping bandwidth;
cyclic shift (CS, cyclic shift) used by the SRS;
comb (comb) used by the SRS; and
the number of antenna ports used for SRS transmission
To notify the above SRS configuration parameters to the terminal, the base station may use the PDCCH to deliver the SRS configuration parameters to the terminal. However, because there is a large amount of information, while resources of the PDCCH are relatively precious, actually the base station may first deliver part of SRS configuration information to the terminal in advance through higher layer signaling (RRC signaling), and deliver the remaining configuration parameters through the PDCCH. In this way, the overhead of the PDCCH is relatively small. For example, the base station may notify the transmission time, frequency domain position, bandwidth, hopping bandwidth, and so on to the terminal through the higher layer signaling, and then notify trigger information (whether to trigger the terminal), cyclic shift information, and comb information to the terminal through the PDCCH.
Downlink control signaling (DCI, Downlink control signaling) refers to signaling transmitted on the PDCCH. DCI may be classified into multiple formats (DCI formats), and each format may bear different control signaling. The DCI of each format may include multiple information elements (IEs, information elements), where each IE carries control information of a different function. DCI of some formats includes SRS configuration information elements (namely, one or more bits for triggering an SRS) used for bearing SRS configuration information, while DCI of some formats does not include SRS configuration information elements and thus has no function of triggering an SRS.
In some application scenarios, in a subframe, an eNB may transmit multiple pieces of DCI to a UE, where at least two pieces of DCI have information elements for bearing SRS configuration information. In this case, how a base station triggers a UE to transmit an SRS to avoid confusion is a problem to be studied.